feat: implement tenant OAuth flow and guest achievements

resources/js/admin/pages/AuthCallbackPage.tsx

@@ -0,0 +1,35 @@
+import React from 'react';
+import { useNavigate } from 'react-router-dom';
+import { useAuth } from '../auth/context';
+import { isAuthError } from '../auth/tokens';
+
+export default function AuthCallbackPage() {
+  const { completeLogin } = useAuth();
+  const navigate = useNavigate();
+  const [error, setError] = React.useState<string | null>(null);
+
+  React.useEffect(() => {
+    const params = new URLSearchParams(window.location.search);
+    completeLogin(params)
+      .then((redirectTo) => {
+        navigate(redirectTo ?? '/admin', { replace: true });
+      })
+      .catch((err) => {
+        console.error('[Auth] Callback processing failed', err);
+        if (isAuthError(err) && err.code === 'token_exchange_failed') {
+          setError('Anmeldung fehlgeschlagen. Bitte versuche es erneut.');
+        } else if (isAuthError(err) && err.code === 'invalid_state') {
+          setError('Ungueltiger Login-Vorgang. Bitte starte die Anmeldung erneut.');
+        } else {
+          setError('Unbekannter Fehler beim Login.');
+        }
+      });
+  }, [completeLogin, navigate]);
+
+  return (
+    <div className="flex min-h-screen flex-col items-center justify-center gap-3 p-6 text-center text-sm text-muted-foreground">
+      <span className="text-base font-medium text-foreground">Anmeldung wird verarbeitet ...</span>
+      {error && <div className="max-w-sm rounded border border-red-300 bg-red-50 p-3 text-sm text-red-700">{error}</div>}
+    </div>
+  );
+}